We also have certain "rules" in our home -- no name calling and no hitting are two of them. I've never hit my kids, so if their squabbles escalate and get physical, I simply remind them, "The rules are, no hitting allowed in this family." They like the security of knowing they will never be hit and do not want this rule to be changed in any way. The same rule can be instituted for teasing. In this way, when discipline is required, it would be for breaking the rule rather than focusing on the younger boy and making him out to be a victim. Of course, there can be no name calling or teasing at all, and that includes you and your husband as well as the kids. This can be a challenge as children get older and do rather dopey things. However, it does create a certain level of respect that is healthy.
I assume you've had your son's eye problem checked out with a physician. A very good friend whose son had a serious problem with a lazy eye recently took him to a doctor in New York. After a series of visits and intensive, daily eye exercises, he has improved dramatically. In fact, the problem is barely perceptible now. I don't know any of the particulars, but perhaps you can talk to your eye specialist about this exercise option.